1. Technical Field
The invention relates to an automatic focusing system, and more particularly, to an automatic focusing system that employs an AF operation of an optical path length difference type for performing an automatic focusing operation based on contrasts of an object image imaged by a plurality of imaging surfaces having an optical path length difference thereamong.
2. Description of the Related Art
As an automatic focusing (AF) operation that is employed in television cameras for broadcasting or business, there is known an AF operation of a so-called optical path length difference type (for example, see JP 2002-365517 A (corresponding to U.S. Pat. No. 6,822,801)). For example, in an automatic focusing system employing the AF operation of the optical path length difference type, a light splitting optical system such as a half-mirror is disposed in a taking lens. An optical path of the taking lens is branched into a main optical path and an AF optical path. In the main optical path, an imaging device of a camera main body (in this specification, referred to as a “video imaging device”) is disposed to record or reproduce video. Thus, the video for record or reproduction are imaged by the video imaging device.
In the AF optical path, a plurality of imaging surfaces of an imaging device(s) for AF (in this specification, referred to as an “AF imaging device(s)”) are disposed so as to have an optical path length difference. There are (i) a case where a plurality of AF imaging devices form the plurality of imaging surfaces may be made in a plurality of AF imaging devices and (ii) a case where a single AF imaging device forms the plurality of imaging surfaces. Object light branched into the AF optical path is split by the light splitting optical system disposed in the AF optical path, and then it is incident on the imaging surfaces of the AF imaging device. As a result, the object image is imaged for each imaging surface, and thus an image signal for each imaging surface can be acquired from the AF imaging device. On the basis of the image signal for each imaging surface acquired in this manner, a contrast of the object image imaged for each imaging surface is provided as a focus evaluation value. Then, the focus evaluation values are compared with each other to detect a focusing state (e.g., in-focus, front-focus, rear-focus) of the taking lens with respect to an imaging surface of the video imaging device. The focus of the taking lens is controlled so as to change the focusing state to an in-focus state.
In television cameras, a ⅔ type CCD is generally used as an imaging device of a camera main body (a video imaging device). On the other hand, when the above-mentioned AF operation of the optical path length difference type is employed, a CCD having the same size as the video imaging device is used as the AF imaging device. This is because a range that is a target of the focusing operation (an AF area) is limited to a predetermined range (e.g., a rectangular range) in a photographing screen of the video imaging device and because the range of the AF area can be changed to a desired position or a desired size.
However, since the ⅔ type CCD has a large size and recent imaging devices (a CCD or CMOS sensor) are miniaturized, there arises a problem that it is different to acquire the ⅔ type CCD.